The use of dielectrics on suitable substrates in the making of multi-layer hybrid microelectronic components is well known in the electronics art. In practice, such substrates have been fabricated from numerous types of materials, including alumina, beryllia, aluminum nitride, and silicon carbide.
The glass compositions used in connection with thick film dielectric compositions are critical in several respects. In firing the paste onto the substrate the vehicle is evaporated or burned off, and the glass is melted and flows over the surface of the substrate to form a homogeneous surface which should be essentially free of pores or bubbles. In practice, this means that the binder must be substantially completely driven off before the glass melts, or it will "bubble" up through the molten glass leaving pores or entrapped bubbles. If on the other hand the melting point of the glass is too high, the resin portion will be driven off but the film will lack structural integrity.
Most thick film multi-layer hybrid circuits are presently produced using precious metal conductors in combination with compatible dielectric compositions. These dielectric compositions generally contain large percentages of lead oxide in the glasses used, and typically have a glass transition temperature (Tg) of about 300.degree. C. to 400.degree. C. Dielectric compositions designed for use with precious metal conductors usually perform well, both physically and electrically, after firing in oxidizing atmospheres.
Such compositions, however, perform poorly when fired in non-oxidizing atmospheres such as nitrogen, which reduces lead or bismuth oxide to metallic lead or bismuth, which in turn often volatilize. The volatile metal may travel down the length of furnaces used to fire the hybrid circuit and condense onto parts in the cooler sections of the furnace. Moreover, the low glass transition temperature of such glass systems enhances the possibility of binder entrapment through premature fluidity during the firing cycle, thereby sealing over the glassy matrix and not allowing the escape of decomposition products, and promotes the potential for reactivity with base metal electrodes, i.e., dissolution and blistering.
Base metal multi-layer circuitry, particularly copper, offers advantages not available with precious metal systems. For example, conductivity that is approximately equal to silver without the solder leaching and migration problems associated with silver conductor systems. The base metal systems, however, also have their own special disadvantages, particularly vis-a-vis the dielectric compositions.
Base metal compositions must be formulated such that (1) the individual components in the glass or glasses used within the system remain stable when fired in non-oxidizing atmospheres, (2) allow for the removal of the organic decomposition products created by the organic binder, and (3) exhibit compatibility or minimal reactivity with the copper or other base metal conductors being used to fabricate the multi-layer circuitry, while still yielding physical and electrical properties comparable to air-fired systems.
To date, commercially available nitrogen firable dielectric systems utilize refractory oxide filled, single glass compositions. However, these all have been found to exhibit excessive porosity which leads to failure with humidity, and entrapped carbon which promotes copper protuberance growth and electrical shorts.